1. Field of the Invention
The present invention generally relates to treatment of contaminated soil. An embodiment of the invention relates to in situ thermal desorption soil remediation of mercury contaminated soil.
2. Description of Related Art
Contamination of soil has become a matter of concern in many locations. “Soil” refers to unconsolidated and consolidated material in the ground. Soil may include natural formation material such as dirt, sand, and rock, as well as other material, such as fill material. Soil may become contaminated with chemical, biological, and/or radioactive contaminants. Contamination of soil may occur in a variety of ways, such as material spillage, leakage from storage vessels, and landfill seepage. Additional public health concerns arise if the contaminants migrate into aquifers or into air. Soil contaminants may also migrate into the food supply through bioaccumulation in various species in a food chain.
There are many methods to remediate contaminated soil. “Remediating soil” means treating the soil to remove soil contaminants or to reduce contaminants within the soil (e.g., to acceptable levels). A method of remediating a contaminated site is to excavate the soil and to process the soil in a separate treatment facility to eliminate or reduce contaminant levels within the soil. Many problems associated with this method may limit its use and effectiveness. For example, dust generation that accompanies excavation exposes the surrounding environment and workers to the soil contamination. Also, many tons of soil may need to be excavated to effectively treat even a small contamination site. Equipment, labor, transport, and treatment costs may make the method prohibitively expensive compared to other soil remediation methods.
Biological treatment and in situ chemical treatment may also be used to remediate soil. Biological and/or chemical treatment may involve injecting material into the soil, such that the material reacts and/or moves contamination within the soil. A material injected during a biological or chemical treatment may be a reactant configured to react with the soil contamination to produce reaction products that are not contaminated. Some of the reaction products may be volatile. These reaction products may be removed from the soil.
The material injected during a chemical treatment may be a drive fluid configured to drive the contamination toward an extraction well that removes the contaminant from the soil. The drive fluid may be steam, carbon dioxide, or other fluid. Soil heterogeneity and other factors may, however, inhibit uniform reduction of contaminant levels in the soil using biological treatment and/or chemical treatment. Furthermore, fluid injection may result in migration of contaminants into adjacent soil.
Soil vapor extraction (SVE) is a process that may be used to remove contaminants from subsurface soil. During SVE, some vacuum is applied to draw air through the subsurface soil. Vacuum may be applied at a soil/air interface or through vacuum wells placed within the soil. The air may entrain and carry volatile contaminants toward the vacuum source. Off-gas removed from the soil by the vacuum may include contaminants that were within the soil. The off-gas may be transported to a treatment facility. The off-gas removed from the soil may be processed in the treatment facility to eliminate or reduce contaminants within the off-gas. SVE may allow contaminants to be removed from soil without the need to move or significantly disturb the soil. For example, SVE may be performed under roads, foundations, and other fixed structures.
Permeability of subsurface soil may limit the effectiveness of SVE. Air and vapor may flow through subsurface soil primarily through high permeability regions of the soil. The air and vapor may bypass low permeability regions of the soil, allowing relatively large amounts of contaminants to remain in the soil. Areas of high and low permeability may be characterized by, for example, moisture, stratified soil layers, and fractures and material heterogeneities within the soil.
Water may be present within soil. At a certain level within some soil, pore spaces within the soil become saturated with water. This level is referred to as the saturation zone. In the vadose zone, above the saturation zone, pore spaces within the soil are filled with water and gas. The interface between the vadose zone and the saturated zone is referred to as the water table. The depth of the water table refers to the depth of the saturated zone. The saturated zone may be limited by an aquitard. An aquitard is a low permeability layer of soil that inhibits migration of water.
Reduced air permeability due to water retention may inhibit contact of flowing air with contaminants in the soil during SVE soil remediation. Dewatering the soil may partially solve the problem of water retention. The soil may be dewatered by lowering the water table and/or by using a vacuum dewatering technique. These methods may not be effective methods of opening the pores of the soil to admit airflow. Capillary forces may inhibit removal of water from the soil when the water table is lowered. Lowering the water table may result in moist soil, which may limit air conductivity.
A vacuum dewatering technique may have practical limitations. The vacuum generated during a vacuum dewatering technique may diminish rapidly with distance from the dewatering wells. The use of vacuum dewatering may not significantly decrease water retention in the soil. This method may also result in the formation of preferential passageways for air conductivity located adjacent to the dewatering wells.
Many types of soil are characterized by horizontal layering with alternating layers of high and low permeability. A common example of a layered type of soil is lacustrine sediments, characterized by thin beds of alternating silty and sandy layers. Attempts to conduct SVE in such layers results in airflow that occurs substantially within the sandy layers and bypasses the silty layers.
Heterogeneities may be present in soil. Air and vapor may preferentially flow through certain regions or layers of heterogeneous soil, such as gravel beds. Air and vapor may be impeded from flowing through other regions or layers of heterogeneous soil, such as clay beds. Also, for example, air and vapor tend to flow preferentially through voids in poorly compacted fill material. Air and vapor may be impeded from flowing through overly compacted fill material. Buried debris within fill material may also impede the flow of air through soil.
Some components of soil contamination may be toxic. Such soil contamination may include mercury, mercury-containing compounds such as dimethyl mercury, radioactive materials such as plutonium, volatile hazardous compounds, and combinations thereof. Placement of wells or use of invasive testing procedures to identify the location and extent of the soil contamination may require special measures to ensure that the surrounding environment and workers are not exposed to contaminated vapor, dust, or other forms of contamination during installation and use of the wells or testing procedures. Such measures may include, but are not limited to, placing dust or vapor producing operations within enclosures to prevent release of contaminants to the environment, treating air within such enclosures to remove or reduce contamination before releasing the air to the environment, equipping workers with appropriate protective clothing, and/or equipping workers with appropriate breathing filters or separate source air supplies.
In some cases, removal of some contaminants from affected soil may be impractical, but removal of other contaminants may be desirable. For example, soil that is contaminated with radioactive material may also be contaminated with other contaminants such as mercury, mercury-containing compounds, hydrocarbons, and/or chlorinated hydrocarbons. Removal of the radioactive material may be impossible or impractical, but it may be desirable to remove or reduce other contaminants within the soil to inhibit such contamination from migrating to other areas through the soil.
The presence of water within the ground is often a problem for construction projects. The problem of water presence and/or water recharge may have to be overcome for some construction projects. A barrier to water migration into a selected area may be established by forming a freeze wall surrounding the selected area. The use of freeze walls to stabilize soil adjacent to a work site and to inhibit water migration into the work site has been implemented during construction of tunnels and shafts and during excavation work. In a typical application of freeze wells at a work site, freeze wells are inserted into the soil and a wall of frozen water and soil is formed around a selected area. The soil within the selected area is then excavated to form a hole. Supports may prevent the walls defining the hole from falling in. The freeze wall may be allowed to thaw when sufficient support is installed to prevent collapse of the walls. Alternatively, work within the hole formed by the removal of the soil may be completed relying on the frozen wall of water and soil to prevent the hole from collapsing. The frozen wall of water and soil may be allowed to thaw after completion of the work within the well.
U.S. Pat. No. 2,777,679 issued to Ljungström, which is incorporated by reference as if fully set forth herein, describes creating a frozen barrier to define a perimeter of a zone that is to be subjected to hydrocarbon production. Material within the zone is pyrolyzed by convectively advancing a heating front through the material to drive pyrolysis products toward production wells. U.S. Pat. No. 4,860,544, issued to Krieg et al., which is incorporated by reference as if fully set forth herein, describes establishing a closed cryogenic barrier confinement system about a predetermined volume extending downward from or beneath a surface region of Earth, i.e., a containment site.
Mercury contamination in soil presents a serious long-term hazard. Instances of widespread health problems resulting from mercury contamination have been documented in many countries around the world. Some mercury contamination is due to spills from industrial sources. For example, mercury spills from vessels that were used as electrodes in chloro/alkali plants are known sources of mercury contamination. Mercury contamination and mercury compound contamination may have occurred at mining and ore processing sites, battery manufacturing facilities, and may also be due to spills, leakage, and/or breakage of barometers, manometers, thermometers, mercury switches, and other mercury containing instruments and vessels. Unacceptable levels of mercury or mercury compounds may also be present in industrial and/or municipal sludge.
Elemental mercury may enter into soil if the pressure head of mercury exceeds the capillary entry pressure of the soil. The mercury may continue to move downward through the soil until the mercury encounters a low permeability layer in which small pore sizes result in high capillary pressures that prevent entry of the mercury. Mercury will typically pass into soil having a porosity greater than about 100 millidarcies. When the mercury reaches a barrier that it cannot pass into, the mercury may flow laterally along the barrier and pool in low places. A portion of a mercury spill that passes through soil may remain within pores of the soil. The amount of mercury retained within the pores of the soil may depend on pore shape and on mercury saturation. Typically, the pore space in a clean sandy soil will hold from 5% to 20% by volume of residual mercury per pore volume of the soil.
The physical properties of mercury may make mercury hard to remove from soil. The density of mercury (13.5 g/cc at 20° C.) may make it difficult to pump mercury out of soil. The retention of a portion of mercury within soil pore space may make it difficult to remove mercury from the soil so that the soil is no longer considered to be contaminated by mercury. The low vapor pressure of mercury (e.g., 0.0012 mmHg at 20° C. and 0.2729 mmHg at 100° C.) may make removal of mercury by a soil vapor extraction process at low or slightly elevated temperatures too time consuming to feasibly remediate mercury contaminated soil.
Mercury contaminated soil may be treated by soil excavation and subsequent treatment of the soil to remove the mercury. Excavated soil may be treated by leaching the mercury from the soil and/or by heating the soil to remove the mercury. Removal, treatment, and transportation of mercury containing soil may not be practical for large contaminated sites. Other types of soil contaminants, such as organic and/or radioactive contaminants, may be present in mercury contaminated soil. Safety considerations due to the presence of mercury and other types of contaminants may weigh against the use of excavation and subsequent treatment of mercury contaminated soil as a remediation method for treating the soil.